Baluns convert between balanced and unbalanced electrical signals and can also provide impedance transformation. Baluns are widely used to couple power transistors such as push-pull or switched power transistors to a single-ended (i.e., unbalanced) 50Ω environment such as a coaxial cable. The balun converts between the balanced output of the power transistor and the unbalanced output line and matches the relatively low drain impedance of the power transistor to the relatively high impedance of the single-ended load. A greater impedance transformation ratio can be realized by coupling two transformers together. Typically, one or both of the transformers include a discrete wire-wound structure such as a coaxial cable wound around a guide or a conductive microstrip structure printed onto a single layer of a PCB (printed circuit board). One transformer is coupled to a single-ended output line while the other transformer is coupled to the power transistor drain. The transformers are conventionally capacitively coupled to the drain of the device by one or more DC blocking capacitors. A similar balun arrangement is used at the input (gate) side of the power transistor. As such, the input and output of the power transistor are capacitively coupled to respective single-ended input and output lines through multistage baluns. The DC blocking capacitors of each balun tend to be small in size. At high power levels (e.g., 300 W or greater), significant heating occurs. Excessively high temperatures destroy DC blocking capacitors, limiting the usefulness of conventional multistage baluns to power applications of about 300 W or less.
Most circuits using conventional multistage baluns also typically have a single-sided DC feed path to the drain of the power transistor. In many applications, the drain of a power transistor has a relatively wide trace so that the drain is low impedance (e.g., 10Ω or less). Providing DC power to the drain of a power transistor through a single-sided DC feed path causes both sides of the drain to be terminated at different electrical lengths, e.g., ¼ at the DC feed path side and ½ at the other side. Single-sided DC feed structures cause unequal terminating impedances and/or high inductance feeding, both of which adversely affect transistor operation. A high inductance feed path to the drain of a power transistor is particularly problematic for high bandwidth applications such as COFDM (coded orthogonal frequency-division multiplexing) video where signal power levels rapidly rise and fall. Under these signal switching conditions, a high inductance feed can cause repetitive L di/dt avalanche breakdown conditions to occur in the power transistor.
It is known to use a single broadside-coupled stripline structure as a transformer in a power amplifier device. A broadside-coupled stripline structure typically includes two ground planes between which one stripline conductor is spaced apart and electromagnetically coupled to a second stripline conductor. However, the single broadside-coupled stripline transformer is still capacitively coupled to a wire-wound transformer or a transformer microstrip structure to complete the impedance matching and balun structure. This type of structure is still prone to excessive DC blocking capacitor heating at high power conditions as explained above, and thus is limited to lower power applications. This type of multistage balun also uses a single-sided path to feed DC power to the drain of a power transistor, causing unequal terminating impedances and/or high inductance feeding.